As a technique of recycling organic waste materials into resources, an attempt has been made to recover phosphorus from activated sludge and put it into resources. For example, in “Digest relating to development and research of recycling systems of organic waste materials into resources for a recycling society (2001)”, a process, wherein activated sludge produced in sewage disposal is treated with heat and then phosphorus is eluted from activated sludge into a liquid phase, is mentioned. In the foregoing digest, as “Recovering of extricated phosphorus by utilizing thermophilic bacteria”, a result of eluting phosphorus from activated sludge into a liquid phase only by treating activated sludge actually utilized in a sewage disposal with heat at 70° C. for 150 minutes, is mentioned (FIG. 9). FIG. 9 shows that as heat treating time becomes longer, the sum of the concentration of total phosphorus in a liquid phase and the concentration of orthophosphoric acid increase steadily, but the concentration of polyphosphoric acid hardly does.
As a result, in a process of only treating with heat, while it takes a very long time of 150 minutes, to elute phosphorus from activated sludge into a liquid phase, the recovery rate of phosphorus is as low as 50%. Also, efficient recovery of phosphorus from a liquid phase cannot be made because the amount of eluted polyphosphoric acid, which coagulates and is recovered more easily than orthophosphoric, is very small.
The object of the present invention is to solve the foregoing problem and to provide a process of eluting phosphorus from sludge into a liquid phase efficiently in a shorter time.
Another object of the present invention is to provide a process of recovering phosphorus after eluting phosphorus from sludge into a liquid phase.